1. Field of the Invention
The present invention relates to a shift device adapted for a transmission in which pushing force applied to a sleeve can be amplified into larger pushing force acting on a synchronizer ring while gears of the transmission are shifted, thereby decreasing operating force necessary for a driver or an actuator.
2. Description of the Related Art
A shift device adapted for a transmission of this kind is disclosed in Japanese Examined Patent Application Publication No. 45-35684. This conventional shift device includes a hub, a sleeve, thrust plates and a synchronizer ring, where the sleeve (or the thrust plates) is formed with slanted surfaces so that the slanted surfaces can change a part of friction torque generated between the sleeve (or the thrust plates) and the hub into thrust acting on the synchronizer ring, thereby increasing synchronizing performance.
In this conventional shift device, the slanted surfaces of the hub need to be formed in the vicinity of a central portion in an axial direction thereof, which causes a problem in that a manufacturing process for forming the slanted surfaces on the hub becomes difficult, consequently increasing its manufacturing costs. Incidentally, the slanted surfaces are indicated by reference number 44 in FIG. 2 and FIG. 17 of Japanese Examined Patent Application Publication No. 45-35684.
In order to remove such a problem, the inventor has suggested a shift device with a hub which has a shape easy to be formed by a sintering method and the like, which is disclosed in Japanese Unexamined Patent Application Publication No. 2006-226515.
This improved shift device, however, has the following problem.
The suggested shift device has the hub in a shape easy to be formed by a sintering method or the like, improving synchronization performance, but it has the same-extent self-servo functions in shift operations to one gear and the other gear arranged at opposite side of the hub. In general, required synchronization performances of shift devices are varied according to transmission gear ratios, namely speed ratios. As a result, there is a problem in that the improved shift device cannot properly accommodate synchronization performances to a shift device that needs to shift between different self-servo functions including only one self-servo function at one-side speed gear. In order to properly accommodate different self-servo functions, six thrust pieces are needed, consequently increasing its manufacturing costs.
It is, therefore, an object of the present invention to provide a shift device adapted for a transmission which overcomes the foregoing drawbacks and can provide different self-servo functions between shift operations to one-side speed gear and the other-side speed gear, decreasing manufacturing costs of the shift device.